Introduction

B1 This appendix sets out the levels of moisture and other pollutants that the provisions in this approved document are designed to control. The provisions are designed to control all of the following.

a. Moisture levels, as described in paragraph B2.

b. Indoor air pollutants, as described in paragraph B4.

c. Bio-effluents, as described in paragraph B6.

NOTE: The guidance in this approved document may not be adequate to address pollutants from flueless combustion space heaters. This approved document does not address the airborne spread of infection and does not directly address contamination from outdoor sources.

NOTE: A strategy for achieving good indoor air quality includes reducing the release of water vapour and air pollutants. This approved document does not provide guidance on such strategies.

Performance criteria for dwellings

B2 The performance criterion for moisture is that there should be no visible mould on the inner surfaces of the external walls of a properly heated dwelling with typical moisture generation.

B3 Mould can grow whether the dwelling is occupied or unoccupied, so the performance criterion for humidity (given in Table B3) should be met at all times, regardless of whether there are occupants. The other pollutants listed in Table B1 are only of concern when the dwelling is occupied.

NOTE: The moisture criteria to avoid house dust mite allergens are more complex and demanding than those to avoid mould. This approved document does not give guidance on the control of house dust mite allergens.

B4 The performance criteria for indoor air pollutants are given in Table B1. These are based on the World Health Organization’s Guidelines for Indoor Air Quality: Selected Pollutants (2010) and Public Health England’s Indoor Air Quality Guidelines for Selected Volatile Organic Compounds (VOCs) in the UK (2019).

Table B1 Indoor air pollutants guidance values(1)(2)

B5 As an alternative to using TVOC, the individual VOCs may be applied where their use is supported by robust independent evidence. Public Health England’s Indoor Air Quality Guidelines for Selected Volatile Organic Compounds (VOCs) in the UK should be used. Testing against these metrics is likely to be more complex than testing against TVOC.

B6 Control of bio-effluents (body odours) for people who have been exposed to the environment for a period of time will be achieved by an air supply rate of 4 litres per second per person (BS EN 16798-1). Assumptions used in applying performance criteria for dwellings in Section 1 General.

B7 Where the guidance for less airtight dwellings is followed, dwellings are assumed to have an infiltration rate of 0.15 air changes per hour.

B8 Where the guidance for highly airtight dwellings is followed, dwellings are assumed to have an infiltration rate of 0 air changes per hour.

B9 Ventilation effectiveness is assumed to be 1.0 – that is, it is assumed that supply air is fully mixed with room air. CIBSE’s Guide A Environmental Design provides further information on ventilation effectiveness.

B10 For the purposes of this approved document, for all dwellings (both new and existing, where Part F applies), the moisture criteria are likely to be met if, during the colder months of the year, the moving average surface water activity of the internal surfaces of external walls is always less than the value in Table B2, evaluated over each moving average period.

Table B2 Surface water activity

B11 For new dwellings, for the purposes of this approved document, the moisture criteria in Table B2 is likely to be met if, during the colder months of the year, the moving average relative humidity in a room is always less than the value given in Table B3, evaluated over each moving average period.

Table B3 Indoor air relative humidity

Appendix B: Performance-based ventilation